Tufts/CfA/MIT Cosmology Seminar, at CfA:

Christine Jones-Forman, Alexey Vikhlinin, and Lawrence David,
 Center for Astrophysics

Title: Hot Gas and Dark Matter in Clusters of Galaxies:
Recent Results on Mass Measurements and Cluster Evolution

Abstract:

Since the suggestion by Zwicky (1933) that clusters of galaxies are
dominated by an unseen component, astronomers have labored to map the
mass distribution in clusters.  X-ray imaging and spectroscopic
observations of the hot intracluster medium are yielding new insights
into how the dark matter in these systems is distributed, how it is
enriched in heavy elements, and what fraction of the total mass is
baryonic.  In cluster cores, the dominant luminous baryonic component
is the hot gas which contains up to five times the mass in the
galaxies and about 30% of the total cluster mass.

There is a clear trend of increasing gas mass to total mass, from
elliptical galaxies, where only 2% or less of their total mass is in
hot gas, through groups and rich clusters where the gas mass fraction
approaches 30%. This result begs the question, where are the baryons
on galaxy scales? We suggest that the baryonic content of galaxies,
groups, and clusters is actually a constant, by using the recent
detection rate of MACHOs in our galaxy.

Earlier reports from EXOSAT observations and the Einstein Medium
Sensitivity Survey suggested that the X-ray luminosities of clusters
evolved over time, such that there were fewer luminous clusters in the
past and that this evolution occurred over relatively modest redshifts
(z < 0.3). To better determine the cosmological evolution of clusters,
we are searching deep X-ray images for spatially extended sources.  We
will discuss the constraints on the cosmological evolution of clusters
based an a sample of 107 X-ray sources detected on the basis on their
spatial entent in 250 deep ROSAT PSPC observations.